Today I went out and fit the two forward bulkheads. These things fit along the side skins vertically and have notches cut out for the aux longeron and stiffener and has tabs that get riveted to the main and lower longerons.

This is the right main gear reinforcement web. This thing nests inside the lower and aux longerons and gets riveted to them and the skin. The gear leg weldment goes in this corner (note the gear leg hole) so the web basically beefs up this area.